Bulbous bow construction of a vessel



y 1969 D. WEICKER 3,455,262

BULBOUS BOW CONSTRUCTION OF A VESSEL Filed Nov. 14, 1967 FIG. I S

CWL

CWL

FCL

CWL

INVENTOR DIETHARD WEICKER FCL United States Patent 3,455,262 BULBOUS BOW CONSTRUCTION OF A VESSEL Diethard Weicker, Rostock, Germany, assignor to VEB Schiifswerft N eptun Rostok, Rostok, Germany Filed Nov. 14, 1967, Ser. No. 682,934 Int. Cl. B63]: 1/40, 1/32 us. or. 114-56 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to a vessel construction and, in particular, to a bulbous bow construction with regard to the designed draught of a vessel.

Bow constructions having different types of bulbous outlines are designed so as to reduce the wave resistance of the vessel by diminishing the waves caused by the vessel and, consequently, to attain correspondingly increased vessel performance, speed or fuel savings. On the other hand, the bulbous shape may produce side effects on the resistance component which is due to viscosity. This may cause either deterioration of the resistance due to flow separation along the bulb or result in an improvement, due to reduced eddy making in case of full fore shapes.

Further possible side effects of a bulbous bow may show in its influence on the water spray formation at the stem and on the vessel behavior during sea going.

With regard to the efficiency and expediency, the known bulbous bow constructions have been designed according to the following rules:

A. The bulbous portion of the bow should be located deeply beneath the construction water line (CWL) of the vessel and have a drop-shaped cross-sectional contour, adjacent to the forward perpendicular construction line (FCL);

B. Worth-while improvements by a bulb are generally attainable only. under light draught conditions of the vessel; and i C. Th improvements will show only at a relatively high I speed length ratio.

These rules, however, are to be considered as physical laws only up to a certain point and, therefore, it is quite possible to extend the field of useful application of bulbous bows. As a matter of fact, such broader application has occurred already in recent years, namely in connection with the speed length ratio; themodern, extremely long freighters and tankers have been almost without exception constructed with a bulbous bow. It is known that due to this bulbous construction in numerous cases at least during ballast travel a remarkable improvement has been attained, in spite of a relatively low speed length ratio. This is partially explainable by the fact that those vessels owing to their large block-coefficient or bluntness and breadth travel at an overpowered condition even at a small Froudes number, that is, in the range of a steeper slope of the resistance characteristic where the wave resistance constitutes an essential part of the total resistance, and

presumably also by the fact that the bulbous bow may reduce eddy making on the bottom of the vessel when the fore has a full contour.

As to the first two rules (A and B), they are interdependent, since the deeper below the CWL the bulb of the bows is arranged, the more reduced are the resulting pressure changes due to this bulb in the region of construction water line (CWL) and, consequently, its influence on the waves. For this reason, a deeply submerged bulb, if designed to improve the wave resistance at the designed draught, should have a size which for other reasons (such as eddy streaming, hydrodynamical impact of the sea) cannot be taken under consideration.

However, it may also occur that the improvements in resistance, caused by the'deep arrangement of the bulb may be more effective for the construction draught than for the ballast draught. This possibility, nevertheless, does not contradict the above mentioned physical interdependence, but it indicates merely that the increase of resistance owing to the eddy and in case of an inconvenient shape of the bulbous portion, may, under certain circumstances offset the improvements in wave resistance.

In principle, from the above discussed effect of the bulbous bow, one may conclude that the most favourable conditions which eifectively influence the wave generation, or improve the wave resistance, arise when the bulb is arranged near beneath the instantaneous travelling water lines. Experience has shown that a partially emerging bulbous portion during zero vessel speed cannot be said to be disadvantageous.

On the basis of this experience, one may conclude that also for the construction draught, substantial improvements may be attained, namely in wave resistance and, by avoiding the eddy making, also in total resistance, if the bulb is arranged closer below the construction water line than it has been customary. Experiments have been made, to find out the influence of the draught on the mode of action of the bulb which has been arranged in the region of the float-water line. As a result of these experiments, a spindle-like bulb (a streamlined rotational body) has been employed having an axis of symmetry somewhat inclined approximately at the level of the construction water line.

A further bulbous bow construction known from prior art is the so called Maierform SV bow wherein the center of gravity of the cross-sectional plane of the fore is located above the center of the construction draught. Its characteristical cross-sectional contour extends upwardly to the construction water line and downwardly, forming a V-like shape, to the base line so that the contour resembles the letter S wherefore the name to this type of bulbous body.

Both the spindle-like and the SV-bow shapes represent an extension of the range of application of bulbous bows beyond the previously mentioned rules A and B.

Nevertheless, there are still several drawbacks associated with the traditional bulbs and, consequently, none of them can be considered as the most effective solution of the problem.

For example, the water-line bulb having a spindlelike configuration has the shortcoming that it does not produce any effect on the wave formation when applied to lower draughts, and further that the spindle-like shape has the tendency to produce flow separation.

The Maierform SV bows exhibit a drawback at ballast draughts because they use the previously described typical stem contour having a V-shaped bulbous crosssection at its lower part. For this reason, it is impossible to adapt the bulb for different float water lines of the vessel. Moreover, this type of bulbous body construction cannot represent the optimum solution even in case of construction draught, which fact can be deduced from general theoretical considerations and being supported by published resistance measurements and by graphical representation of the wave making of a body of rotation towed at different depths or draughts (see for instance 3 Naval Engineers Journal, 77 (1965), No. 6, pg. 943, A. Nutku: Waterline Bulb on High Speed Boats).

Therefore, the primary object of this invention is to avoid the aforementioned drawbacks of prior art bulbous bows.

More particularly, the object of the present invention is to provide a bow the bulb of which forms a shallow shore from which the water, instead of being swelled on the stern, softly flows aside without formation of spraying water, thus most efliciently eliminating the bow wave.

According to the present invention these objects are attained so that the bulb extends from the base line of the ship upward to an essential height above the designed water line (CWL).

In other words, the bulbous bow of the invention comprises a concave or recessed section the vertex of which is defined by a distance a aft of the designed perpendicular forward construction line and a distance b above the CWL, and a convex or protruding section integral with the concave section, the vertex of the convex section being defined by a distance below the construction water line and a distance d forward of the perpendicular forward construction line, whereby a, b, c, and d are essentially greater than zero.

The dimensions of respective distances a, b, c and d are different for different shapes and desired speeds of the vessel, however, in accordance with the bulbous bows of this invention, the contour and the water-line offsets of the bulbous bow are adapted at its upper portion to the shape of the vessel and to the speed at the designed draught thereof and, at its lower portion, to the respective conditions of the vessel in ballast passage.

The manner in which the objects of the invention are accomplished will be explained in greater detail below with reference to the following drawings, wherein FIG. 1 is a side view of a contour of the bulbous bow according to this invention in application for vessels having small draught differences;

FIGS. 2 and 3 show a side view of the bulbous bow of FIG. 1 modified for vessels having increased draught variation and different frame shapes.

Referring now to the figures, the stem S of a vessel is defined for design purposes, by a construction water line CWL, base line BL and perpendicular forward, construction line FCL. The dotted line contour 2 denotes the stem construction without bulbous configuration whereas the solid line 1 illustrates the bulbous bow of this invention.

Line 1 extends from the base line BL beyond the forward construction line to form a convex section (hatched) having a vertex V which is defined by its distance d from the FCL and c from the CWL. This protruding or convex section recesses upwardly to a substantial height b above the CWL and back aft or the FCL to form an integral concave or receding bulb section (hatched) having a vertex V defined by a distance a from FCL. The magnitude of distances c and d are dependent on the form of the frame and on designed draught differences for the vessel, influencing the float properties during ballasted travel. The contour of the stem and of the bulb depends both on the shape of the vessel and on the designed speed at construction draught.

Since a wave at the bow of a vessel is generated as a consequence of positive stream pressures resulting at the bow because of the speed of the vessel, it is evident that the bulbous how will fulfill its purpose, namely a complete elimination of bow wave and, of free waves caused by the bow wave, if it more or less compensates those stream overpressures by means of its own underpressure field. The effect of the bulbous body is most favourable when this pressure compensation is as complete as to entirely eliminate the resulting bow waves.

In practice, however, such a condition can never be fulfilled. For this reason, the bulbous bow construction is indicated as most favorable when the underpressures generated by the bulb have the same magnitude as the overpressures generated by the vessel without bulbous body (provided that the bulbous body is suitably arranged in the longitudinal direction). Due to the fact that generation of waves depends mainly on stream pressures generated closely at the water surface it results further, that the under pressures generated by the bulbous bow in the region of the float water line should have the same magnitude as the overpressures generated at this region by the vessel. As known, the stream pressures on a body are due to a deflection of momentum of flow. Accordingly, the overpressure magnitude at the bow, and thereby the height of the bow wave of a (bulbless) vessel, is defined for a predetermined speed by the entrance angle of the float water line, which angle in turn is directly proportional to the breadth of that float water line in the region of the bow wave. In case of a bulbous body floating independently on the vessel, the breadth of the float water line of that body, required for generation of an underpressure field of that approximately equal magnitude, may have reduced dimension because of increased curvature of its wall stream line. Notwithstanding, it represents at any event an essential percentage of the breadth of the float water line of the vessel in the region of the bow wave, since in practice the sides of the bulbous body can be curved only within very narrow limits because otherwise the generation of underpressure might be impaired due to flow separation.

It follows from the above that the bulbous bow must have a definite breadth in the region of the respective floatwater lines (a substantial breadth with respect to that of the float water line of the vessel in the region of the bow wave) which feature was not met by the bulb configurations known from prior art.

In practice, the selection of the breadth of the bulb cannot be accomplished by considering the wave resistance exclusively, but also the necessity how to avoid forming of eddy streaming and water spraying must be taken into account.

The formation of spraying water at the stem is the lower the more reduced is the angle at which the uppermost water layers of the bow wave impact against the stem. With regard to this fact, it is obvious that the bulbous section which extends above the construction water line, approximately as high as the wave of the bow of a bulbless vessel, is most favourable for the vessel floating at construction draught.

In this case the water of the bow wave, which has been shifted by the bulb in forward direction, may at least partially keep flowing away by the sides, before being piled up against the perpendicular portion of the stem. Since the formation of spraying water likewise increases the resistance the reduction of spraying water results not only in a better seagoing behaviour but also in further power savings.

The advantages of the bulbous bow which, according to this invention, is not entirely submerged during the zero speed of the vessel, are the more apparent the blunter is the respective float water line of the vessel. Consequently, increased savings may be attained by means of the present bulbous bow, even if no provisions are made for adjusting the bulbous shape in the lower region under water at the ballast condition of the vessel.

With respect to the aforementioned considerations, it is possible to yield still greater savings by means of the present bulb, because it can be designed at the same time for all important floating conditions of the vessel.

As a consequence of great diversity of possible shapes and speeds of the vessel, there is likewise a great diversity in respective dimensions a, b, 0 and d as well as in configuration of cross-sectional planes of the bulbous bow. In any case, a height 12 essentially greater than zero is a necessary presumption of the maximum efliciency of the bulbous bow according to the present invention. The actual scope of this invention does not relate therefore, to any specific vessel, but it should be determined by the attached claims.

What is claimed is:

1. In a vessel defined by a designed construction water line (CWL) and a designed perpendicular forward line (FCL), a bulbous bow comprising a recessing upper portion integral with a lower portion protruding forward of the perpendicular forward line below the designed construction water line, the vertex of said upper portion being defined by a distance a aft of said perpendicular forward line and by a distance b above said construction water line, said distance b being of the magnitude of bow waves of a bulbless vessel.

References Cited UNITED STATES PATENTS 2/ 1967 Eckert 114-56 1/1968 Kaname Taniguchi 114i6 ANDREW I-I. FARRELL, Primary Examiner 

